It depends on your hair type, if you have weaker already damaged hair or thinner hair then regular flat ironing can cause even more damage. If you have healthy hair, use a protectant and take good care of your hair and it can handle more flat ironing then go for it.
I noticed the biggest Improvement in my hair when I stopped flat ironing it every week. My hair grew so much and it felt so much better. It also looks a lot better when I do flat iron.
I flat iron about 2-3 times a year and that is in the winter months. We get hot humid weather all summer and it is a waste of my time to flat iron then since it doesn't stay straight in humidity or when I sweat. It's also easier for me to keep it curly because I go to the gym a lot.
I know it's hard to pick up the flat iron when you have worked so hard taking care of your hair but it doesn't have to be all bad if you use the lowest heat setting possible to achieve straight hair for you and you use a heat protectant and add the moisture back in when you wash and condition it should be okay once in a while.